I wear sunglasses a lot. This may sound a bit strange, but I really want to be able to see well when I'm hanging out in beautiful places. Half the reason I'm there is to be able to look at things. Doing what I do it's unlikely I'll get hit in the face by a stone... Here's what I factor in:
(1) Optical quality. Here you do get what you pay for, even if you're also paying for fashion. I've had a pair of Ray-Bans just once: awesome to look through! I'd have another pair instantly, if I could afford them. Many have mentioned Oakleys, also nice.
(2) Tint. For me, it has to be neutral density (grey); this doesn't change the colour of what you're looking at, just makes it darker. In the winter, a gentle amber tint (the old 'smoked' lenses) can warm things up a bit, but I have no idea how people can go around looking through yellow and pink and blue. It makes me feel disconnected from what I'm looking at, like it's on a screen.
(3) Polarisation. The world just looks SO much better through polarising lenses. The sky, water, foliage, everything. Quality polarisers make things fizz.
Currently I have a pair of 'Dirty Dawg' polarisers. Picked them up going through Newquay on the Coast Path last summer, after losing my previous pair. Plenty of sunglasses to choose from in Newquay! And all terribly trendy of course. I spent as much as I had. At £60 they were cheaper than the Oakleys but still seemed as nice to look through. Still very happy with them, and they're holding up fine, although some of the 'paint' has peeled on the arms where DEET's got on them.